Tumors are immunogenic and are not rejected. Studies with sponge matrix allografts have shown that a foreign grafts requires specifically sensitized cells to accumulate at the site of the antigen in sufficient quantity to cause rejection. Effective immunity is amplified at the site of graft rejection both by maturation of precytotoxic cells and their recruitment from immunized lymphoid depots of sensitized cells which are induced to migrate into the circulation and then accumulate at the site of rejection. Migration of sensitized cells to the graft requires, in turn, a specifically sensitized cell capable of manufacturing a lymphokine which will recruit further cells from the circulation in a nonspecific manner. The present studies are designed to characterize the cells infiltrating allografts and the mechanisms by which amplification occurs at the graft site. Emphasis in the current research has turned from the allograft situation to the syngenic tumor graft. We will be attempting to determine whether parallel events occur in tumor grafts and whether natural killer (NK) cells can generate lymphokines required to amplify the immune response at the site of the tumor.